1. Field of the Invention
The disclosures herein relate to a capacitor device having a ferroelectric layer, a semiconductor device having such a capacitor device, and a method of manufacturing such a capacitor device.
2. Description of the Related Art
A ferroelectric layer having a spontaneous polarization is widely used in capacitor devices that serve as decoupling capacitors or that constitute part of semiconductor devices such as DRAM (Dynamic Random Access Memory), for example.
When such capacitor devices are to be made in small size, the area size of the ferroelectric layer needs to be made small. The amount of electric charge stored in a memory device is proportional to the area size of the capacitor devices. When the memory device is to be made in small size or is to be configured to have a large memory volume, therefore, the relative permittivity of the ferroelectric layer needs to be made so large as to increase the volume density of the dielectric layer.
Examples of metallic oxide for use as the ferroelectric layer of a capacitor device include a dielectric material having a perovskite structure such as a barium strontium titanate (BaSrTiO3) (see Japanese Patent Post-Grant Publication No. 8-18867).
Important factors to be considered in order to increase the relative permittivity of such ferroelectric layer include the crystalline characteristics of the ferroelectric layer and the controlling of stress applied to the ferroelectric layer formed in a thin film shape. Treatment temperature at the time of forming a ferroelectric layer may be increased to improve the crystalline characteristics of the ferroelectric layer.
An attempt to increase the temperature for treating a ferroelectric layer may give rise to various problems as follows. An increase in treatment temperature may increase the dielectric loss of the ferroelectric layer. Further, an increase in a leak current through the ferroelectric layer may be brought about, resulting in less reliability as a capacitor device.
Moreover, a high treatment temperature may create strong pulling stress applied to the ferroelectric layer due to a difference in thermal expansion between the substrate made of Si, for example, and the ferroelectric layer. Such stress may create large in-plane deformation in the crystal. If the in-plane deformation of crystal forming a ferroelectric layer becomes large, a decrease in relative permittivity and an increase in leak current are brought about, resulting in the deterioration of electrical characteristics of the ferroelectric layer.
Such stress generated in the ferroelectric layer may be relaxed by selecting a substrate that is thermal expansion matched with the dielectric layer or for epitaxial ferroelectric layer growth having a crystal structure that is lattice-matched with the crystal of the ferroelectric layer. When the selection of a substrate is utilized to relax the stress of a ferroelectric layer, however, a choice of a substrate usable for the forming of a ferroelectric layer will be limited. This method is thus not practical.
What is needed is a capacitor device, a semiconductor device, and a method of making a capacitor device that substantially obviates one or more problems caused by the limitations and disadvantages of the related art.